The transmission of electromagnetic energy through a medium such as the atmosphere has numerous applications. Perhaps the most common is in the field of communications, where information is modulated onto the transmissions between a transmitter and receiver. Another example is radar, or radio direction and ranging, which involves the transmission of a pulse of radio frequency (RF) electromagnetic energy into the atmosphere and the subsequent reception and analysis of RF electromagnetic energy reflected by surrounding objects.
The primary device used to transmit and receive electromagnetic energy is the antenna. When an RF electric signal is applied to an antenna, RF electromagnetic energy is emitted by the antenna and will propagate through the atmosphere. Similarly, the antenna generates an RF electric signal as an output when it receives RF electromagnetic energy.
In many applications, it is desirable to scan the transmitted and received beams of electromagnetic energy. Traditionally, scanning was accomplished by mechanically rotating the antenna. Mechanical scanning arrangements are, however, relatively bulky, slow, and subject to mechanical failure.
As an alternative to mechanically rotatable antennas, phased-array antennas were developed including a plurality of transmit/receive (T/R) modules. Although the relative positions of the modules are mechanically fixed, the modules are electrically controlled to transmit and receive signals along a steerable beam. Specifically, by adjusting the phase and amplitude of the signals applied to, or received from, the various individual modules, the direction of the beam produced or received by the array as a whole can be controlled.
In active phased-array antennas, each module contains at least one antenna element, as well as components for amplifying the signals applied to, and received by, the antenna. Typically, these components also control the phase and amplitude of the signals to effect steering of the antenna beam. Thus, the modules may include amplifiers, phase shifters, circulators, switches, limiters, and other components and, as a result, are typically relatively complex and expensive.
To complete the radar system, the various modules included in the array are coupled to a central processing system. The central processing system generates the signals to be transmitted by the array and interprets the received signals to determine the range, direction, and identity of surrounding objects.
The signals are communicated between the modules and the central processing system by a corporate feed network. Traditionally, corporate feed networks included stripline, waveguide, or coaxial transmission lines to transmit electric signals to and from the modules. Such networks, however, are typically relatively heavy, bulky, and subject to the effects of internal electromagnetic interference (EMI) and external electromagnetic pulses (EMP). It has been discovered that the use of optical fibers in corporate-feed networks overcomes most of these disadvantages. Specifically, fiber-optic feeds are extremely lightweight, compact, and immune from the effects of EMI and EMP.
Fiber-optic feed networks are, however, not without problems. For example, the amount of power that can be optically transmitted by a fiber is typically much less than can be electrically transmitted by a stripline or coaxial transmission line. So, to be useful, an optically transmitted signal must be amplified at the module. As a result, the reduced weight, bulk, and expense of a fiber-optic feed network may be set off by the increased cost and complexity of each module. In view of these observations, it would be desirable to provide a relatively simple and inexpensive T/R module for use in an optically fed, phased-array antenna.